1. Field of the Invention
The present invention relates to apparatus for the measurement of optical path length changes between two plane mirror surfaces. More particularly, the invention relates to optical apparatus which is useful for high accuracy displacement metrology using interferometry.
2. The Prior Art
An interferometer is a basic instrument for most high accuracy displacement measurements in dilatometry, material stability studies, the machine tool industry, and in the semiconductor fabrication industry. One type of interferometer representative of the current state-of-the-art is the differential plane mirror interferometer which measures the optical path length changes between two external mirrors and which is described in R. R. Baldwin and G. J. Siddall, "A double pass attachment for the linear and plane interferometer," Proc. SPIE, Vol. 480, pp. 78-83 (May 1984) and in commonly owned U.S. Pat. Nos. 4,693,605 and 4,752,133 issued Sept. 15, 1987 and Jun. 21, 1988, respectively.
A limit on the accuracy of a heterodyne interferometer is the periodic, i.e. cyclic, error caused by leakage of the frequency components in the beamsplitter, see for example, N. Bobroff, "Residual Errors in Laser Interferometry from Air Turbulence and Non Linearity," Applied Optics, Vol. 26, pp. 2676-2682 (Jul. 1987) and C. M. Sutton, "Non-Linearity in Length Measurement Using Heterodyne Laser Michelson Interferometry," J. Phys. E: Sci. Intrum. Vol. 20, pp. 1290-1292 (1987).
For differential plane mirror interferometers of the prior art, the magnitude of this cyclic error ranges from .+-.1 to .+-.10 nanometers.
The present invention retains the basic plane mirror interferometer cavity of the prior-art differential plane mirror interferometer; however, the means used in the instant invention to produce the reference and measurement beams reduces the magnitude of the cyclic error by 2-3 orders of magnitude, i.e., to .+-.0.001-0.01 nanometer. The improvements of the present invention thusly increase the accuracy which can be attained with this type of interferometer.